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This feature detects a connection failure caused by black hole routers and helps to recover such connections.
Whenever a router receives a datagram with a Don't Fragment (DF) bit set in its header and the packet size is greater than the next MTU, the router cannot forward the packet. In such a case, the router sends an ICMP Destination Unreachable DF bit set message to the host.
Often, routers do not send such a message. Instead they ignore the datagram. Typically, an IP datagram cannot be forwarded because its maximum segment size is too large for the receiving server and the Don't Fragment bit is set in the header of the datagram. Routers that ignore these datagrams and send no message are called PMTU black hole routers. Some routers might silently drop large frames, even when the DF bit is not set. Firewalls are often misconfigured to suppress all ICMP messages.
To respond effectively to black hole routers, the Novell TCP/IP stack provides a Path MTUBH Detect feature. Path MTUBH Detect recognizes repeated unacknowledged transmissions and responds by turning off the Don't Fragment bit. After a datagram is transmitted successfully, the MTUBH Detect feature reduces the maximum segment size and turns the Don't Fragment bit on again.
The feature specifies the maximum transmission unit size of an interface. Each media type has a maximum frame size that can't be exceeded. The Link layer is responsible for discovering this MTU and reporting it to the protocols above it.
For more information, see Path MTU Black Hole Detection and Recovery.
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